1. Field of the Invention
The present invention relates to an image signal processing apparatus, and more particularly to an apparatus for processing an analog color image signal after digitalizing it.
2. Related Background Art
An image signal processing apparatus for processing an analog image signal after digitalizing it has a structure such as shown in FIG. 1.
In FIG. 1, reference numeral 1 represents a clamp circuit, 2 represents an A/D converter, 3 represents a digital signal processing circuit, 4 represents a D/A converter, and 5 represents a variable voltage source. The operation of this apparatus will be described below.
Referring to FIG. 1, an analog image signal inputted from an input terminal a is clamped to a clamp level by the clamp circuit 1 at the timing when a clamp pulse is entered from an input terminal b. The clamp level is preset by the variable voltage source 5.
The analog image signal clamped by the clamp circuit 1 is converted into a digital data by the A/D converter 2 synchronously with a clock pulse inputted from an input terminal c, and thereafter the digital data is supplied to the digital signal processing circuit 3.
The digital signal processing circuit 3 is supplied with the clock pulse inputted from the input terminal c as well as a blanking pulse inputted from an input terminal d. The digital signal processing circuit 3 converts the digital data supplied from the A/D converter 2 into a data which indicates "0" during the blanking period designated by the blanking pulse, and performs various digital signal processing. The processed signal is supplied to the D/A converter 4.
The digital data processed by the digital signal processing circuit 3 is converted into an analog image signal synchronously with the clock pulse inputted from the input terminal c, and thereafter the analog image signal is outputted from an output terminal e.
In the image signal processing circuit shown in FIG. 1, a so-called black level adjustment is carried out by adjusting a preset voltage level of the variable voltage source 5 and changing the clamp level so as to make the blanking level coincide with the black level of an inputted image signal. However, even if the black level of an inputted image signal becomes lower than the blanking level, the digital data outputted from the A/D converter 2 will not lower below "0", resulting in a hardship of adjusting the black level.
Furthermore, even if the minimum reference potential of the A/D converter 2 is arranged to be adjusted so as to match the level of an image signal to be inputted to the A/D converter 2, an error is likely to occur and the circuit becomes complicated, resulting in a hardship of adjusting the black level.
An image signal processing apparatus for processing an analog color image signal after digitalizing it has the structure such as shown in FIG. 2.
In FIG. 2, reference numeral 11 represents a clamp circuit, 12 represents an A/D converter, 13 represents a digital signal processing circuit, 31 and 35 represent switches, 32 represents a luminance matrix circuit, 33 represents a chrominance matrix circuit, 34 represents an adder, 14 represents a D/A converter, and 15 represents a variable voltage source.
The operation of the apparatus shown in FIG. 2 will be described below.
Referring to FIG. 2, analog R, G, and B signals inputted from input terminals are clamped to clamp levels by the clamp circuits 11 at the timing when a clamp pulse is entered from an input terminal. The clamp levels are preset by variable voltage sources 15.
The analog R, G, and B signals clamped by the clamp circuits 11 are converted into digital data by the A/D converters 12 synchronously with a clock pulse inputted from an input terminal, and thereafter the digital data is supplied to the digital signal processing circuit 13.
The digital signal processing circuit 13 is supplied with the above-described clock pulse as well as a blanking pulse inputted from an input terminal. The digital signal processing circuit 13 converts the digital data supplied from the A/D converters 12 into data which indicates "0" during the blanking period designated by the blanking pulse by connecting the switches 31 controlled by the blanking pulse to B terminals shown in FIG. 2. The luminance matrix circuit 32 and chrominance matrix circuit 33 generates a digital luminance signal and R-Y and B-Y digital chrominance signals using the inputted digital data. The digital chrominance signals are supplied to the D/A converters 14 via switches 35 which are controlled by an adjustment signal supplied from an input terminal and connected to D terminals shown in FIG. 2 during the black level adjustment. During the period other than the blanking period, the digital luminance signal is supplied to the D/A convert 14 via the adder 34 and switch 31 controlled by the blanking pulse, the digital luminance signal being added with a data (e.g., a digital data representative of a voltage level "7") set by an offset data a entered from an input terminal so that the black level of the luminance signal relative to the blanking level is adjusted.
The digital luminance signal, R-Y and B-Y digital chrominance signals processed by the digital signal processing circuit 13 in the above manner are converted into analog image signals synchronously with the clock pulse inputted from the input terminal, and thereafter the analog image signals are outputted from output terminals.
In the image signal processing circuit shown in FIG. 2, during the black level adjustment, the R, G, and B digital signals indicating "0" during the blanking period are supplied to the D/A converters 14 by changing the connection of the switches 35 by the adjustment signal, and converted into analog image signals to be outputted. The voltage setting levels of the variable voltage sources 15 are adjusted by the levels of the outputted analog R, G, and B signals to thereby change the clamp levels. In this manner, the blank level is made to become coincident with the black levels of the analog R, G, and B signals inputted 1 from the input terminals, to thereby conduct the black balance adjustment between the luminance signal outputted from the luminance matrix circuit and the chrominance signals outputted from the chrominance matrix circuit 33.
As described above, in the image signal processing apparatus shown in FIG. 2, the voltage setting levels of the variable voltage sources 15 are adjusted to change the clamp levels such that the blanking level is made to become coincident with the black levels of the inputted analog R, G, and B, thereby conducting the black balance adjustment between the outputted luminance signal and analog R-Y and B-Y chrominance signals. However, even if the black levels of the inputted analog R, G, and B signals become lower than the blanking level during the black balance adjustment because of setting particular clamp levels, the digital data outputted from the A/D converter 12 will not lower below "0", resulting in a hardship of adjusting the black level.
Furthermore, even if the minimum reference potentials of the A/D converter 12 are arranged to be adjusted so as to match the levels of analog R, G, and B signals to be inputted to the A/D converters 2, an error is likely to occur and the circuit becomes complicated, resulting in a hardship of adjusting the black level.